The present invention relates to in vivo measurement. More specifically, the present invention relates to sensing, and sensors for sensing, the concentration of particular substances in body fluids.
Measurement of the concentration of particular chemicals in body fluids is useful for many types of medical diagnosis and treatment. Insulin-dependent diabetic patients, for example, might measure the concentration of glucose in their blood multiple times per day, and electrochemical sensors for in vivo measurements of glucose are known. Such sensors typically have a portion that can be inserted into tissue and include one or more electrodes that come into contact with interstitial fluid after insertion. Electronic circuitry external to the human is used to control operation of the sensor by sending electrical signals to sensor electrodes and monitoring an electrochemical reaction that takes place between the fluid/tissue and the electrodes.
One problem with in vivo sensors that are used for continuous monitoring is that the physical and/or chemical characteristics of the sensor as well as the histology of the tissue surrounding the implantation site change over time. For example, the capacitance of the sensor may change, an electrically conductive path of the sensor may fail, or the permeability of one or more membranes in the sensor can change. Similarly, migration of leukocytes and fibrin depositing in the environment containing the analyte can change the electrical conductivity of the environment of the sensor, for example. U.S. Publication No. 2008/0214910, which is hereby incorporated herein by reference, teaches applying an AC input signal to the electrodes of a sensor and monitoring an AC output signal thereby produced. A complex impedance of the sensor based on these AC signals can be determined, from which information concerning the changing characteristics of the sensor can be determined. Sensor performance can thus be improved.
In spite of improvements such as disclosed by U.S. Publication No. 2008/0214910, in vivo sensors remain limited in durability, accuracy, ease of manufacture, and potential lifetime in use. There is thus a need for improved in vivo sensors and sensing techniques.